The weight of a body is maximum in :

vacuum

According to Archimedes’ principle, when a body is immersed in a fluid completely or partially, there is some loss in weight of the body due to buoyant force action on the opposite direction of gravitational force.
So, the weight of the body is = Gravitational force - Buoyant force
In vacuum, there will be no buoyant force. So, the weight of a body will be maximum in vacuum.